BOSTON — A pair of Boston teenagers charged with fatally shooting a 16-year-old as he rode his bicycle through a rough part of the city pleaded not guilty Monday to charges after their mothers turned them in to police over the weekend.

More than a dozen relatives of the pair — Dushawn Taylor-Gennis, 16, and Raeshawn Moody, 14 — packed into the courtroom and wept as the two were charged as adults with murder and firearms violations.

The pair appeared to be waiting for someone in the minutes before last week’s shooting of Jonathan Dos Santos, 16, said Assistant District Attorney Jennifer Hickman, citing surveillance video. Hickman said the video shows the teens separating shortly before the shooting with one of the two appearing to hide before Dos Santos rode up on his bicycle, when he jumped out, brandished a weapon and opened fire.

She did not say which of the two teens pulled the trigger.

The wounded Dos Santos collapsed shortly after the shooting and was later pronounced dead at a hospital.

Prosecutors have said the three teens knew one another but have not disclosed the motive of the shooting.

“The facts as we understand them are chilling,” Suffolk County District Attorney Daniel Conley said in a statement. “We’re still in the early stages of this investigation and every piece of evidence matters.”

The two were ordered held without bail.